The present invention relates to a method and apparatus for alarming decrease in tire air-pressure and a threshold changing program. More particularly, it relates to a method and apparatus for alarming decrease in tire air-pressure and a threshold changing program with which it is possible to detect decompression of a tire and to issue an alarm to a driver.
A system for alarming decrease in tire air-pressure (DWS) in which decompression of a tire is detected on the basis of rotational (wheel speed) information of four wheel tires is conventionally known. Such a system employs a theory that a rotational velocity or a rotational angular velocity of a decompressed tire is increased when compared to remaining normal tires owing to a decrease in outer diameter (dynamic load radius of the tire) from that of a tire of normal internal pressure. For example, in a method for detecting decrease in internal pressure on the basis of a relative difference in rotational angular velocities of tires,DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is employed as a judged value DEL (reference should be made to Japanese Unexamined Patent Publication 305011/1988). Here, F1 to F4 denote rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
It is judged that decompression of a tire is present when a value, which is obtained by calculating the above judged value DEL and performing necessary corrections such as cornering correction and rejections of unnecessary data, has exceeded a preliminarily set threshold, and an alarm is accordingly issued to the driver.
It is also known for an apparatus for alarming decrease in tire air-pressure which detects conditions of air-pressures of tires to issue alarm by extracting a resonance frequency of an oscillation frequency based on the wheel speed pulse of the vehicle and by comparing a decreased deviation which is based on the resonance frequency with a specified deviation (reference should be made to Japanese Unexamined Patent Publication 133831/1993).
In a method for detecting decrease in internal pressure on the basis of relative differences between rotational angular velocities of the tires, the thresholds will be inherent to the tires so that they need to be selected to correspond to kinds of tires such as summer tires, winter tires or run-flat tires with which it is possible to temporarily continue running even in the presence of a burst.
However, in the case of presently performed tunings, the threshold needs to be fixed, and it is thus set to be an intermediate value of the summer tires and winter tires since an alarm should be made even if the tires are exchanged from standard tires to studless tires. It might therefore happen that inconveniences such as erroneous alarm or no or delayed issuance of alarm even in the presence of decompression are caused.
On the other hand, in the apparatus for detecting conditions of air-pressure of tires on the basis of the resonance frequency of the oscillation frequency and accordingly issuing alarm, in the presence of a decompression by 30% in a studless tire and a run-flat tire, the frequency characteristics of the studless tire will be largely changed while the frequency characteristics of the run-flat tire will hardly be changed. Problems such as erroneous alarm are accordingly caused when setting the threshold as discussed above.